The present invention relates to an improved multibeam electron gun for a cathode-ray tube and particularly to an electron gun having improved stability between a plurality of cathode assemblies and two adjacent grid electrodes.
Electron guns, such as used in shadow mask type color picture tubes, are designed to generate and direct preferably three electron beams along convergent paths to a small area of convergence near a screen of a tube. Two general types of guns in most common use are the inline electron gun, wherein three beams are initiated at three points in a line and the delta electron gun, wherein three beams are initiated at the points of a triangle. Each of these types has three separate cathode assemblies and a series of electrodes spaced therefrom. The cathode assemblies and electrodes are held in place relative to each other by separate attachment to a plurality of glass rods. The electrode closest to the cathode assemblies is called the G1 and is usually a control grid. The next electrode is called the G2 and is usually a screen grid. The spacings between the cathodes and these two grids, as well as between the grids themselves, are very critical. For example, a change as small as 0.001 inch (0.025 mm) in the spacing between a cathode and the G1 may change the cutoff voltage of the electron gun by about 60 volts. Unfortunately, during warmup of a tube having a gun construction as previously described, the spacings between the cathodes, G1 and G2, vary to some extent. This variation in spacings causes unstable and nonuniform cutoff voltages for each of the beams in a gun thereby changing the colors which appear in the tube screen. This nonuniformity between beams requires additional circuitry for correction. It is desirable to develop a tube wherein any changes in the spacings between the cathode assemblies and the G1 and G2 electrodes during tube warmup are uniform.